Process for foaming thermoplastic elastomers

ABSTRACT

A foamed article is made by infusing the article of thermoplastic elastomer with a supercritical fluid, then removing the article from the supercritical fluid and either (i) immersing the article in a heated fluid or (ii) irradiating the article with infrared or microwave radiation.

FIELD OF THE INVENTION

The invention is related to methods of making foams and elastomericfoams.

INTRODUCTION TO THE DISCLOSURE

This section provides background information related to this disclosurebut which may or may not be prior art.

Polyurethane foams are typically prepared by using chemically actingblowing agents or physically acting blowing agents that are mixed intoor injected into the monomer reactants during polymerization. Chemicalblowing agents include compounds that form gaseous products by reactionwith isocyanate, for example water or formic acid, while physicalblowing agents are dissolved or emulsified in the monomers and vaporizeunder the conditions of polyurethane formation. These are, for example,hydrocarbons and halogenated hydrocarbons or gases such as carbondioxide, which are introduced either on-line, i.e. directly into themixing head, or via the stock tank in batch operation. Such a process isdescribed, for instance, in Bruchmann et al., US Patent ApplicationPublication No. US 2011/0275732.

Takemura et al., U.S. Pat. No. 6,878,753 describes shoe soles andmidsoles made of a thermoset polyurethane foam. The foam is made by aprocess comprising mixing a polyol solution, which is previouslyprepared by mixing a polyol, with a catalyst, water and urea, a chainextender, and an additive as occasion demands, with a polyisocyanatecompound with stirring in a molding machine; and injecting the resultingmixture into a mold and foaming the mixture. The density of a moldedarticle of the polyurethane foam is said to be 0.15 to 0.45 g/cm³.

Fischer et al., WO 94/20568, describes thermoplastic polyurethanemini-pellet or bead foams with an average diameter of 1-20 millimeters.The polyurethanes are polyester- and polyether-based materials. The beadfoams are molded under pressure and heated by introducing pressurizedsteam.

Prissok et al, US Patent Application Publication No. 2010/0047550describes a hybrid material with a matrix of polyurethane and foamedparticles of thermoplastic polyurethane embedded in the matrix. Thehybrid material may be used for making shoe soles. The matrixpolyurethane may be foamed during molding.

Prissok et al., US Patent Application Publication No. 2010/0222442describes an expandable thermoplastic polyurethane including a blowingagent and having a Shore hardness of A 44 to A 84. Foams can be producedfrom expanded beads of the polyurethane by fusing them to one another ina closed mold with exposure to heat. Prissok et al. teach that the beadsare charged to the mold, the mold is closed, and steam or hot air isintroduced into the mold to further expand the beads and fuse themtogether. A foam made in this way is said to have a density in the rangeof from 8 to 600 g/L.

Nadella, US Patent Application Publication No. US 2010/0052201 describesmaking foamed polymeric panels from solid monolithic semi-crystallinethermoplastic material sheets, such as polylactic acid, polyethyleneterephthalate, polyethylene naphthalate, polybutylene terephthalate,polypropylene, and polyethylene. The disclosed applies to film-typematerials.

A need remains for improved methods of forming foams that can becustomized for cushioning in footwear, protective wear, and similarapplications.

SUMMARY OF THE DISCLOSURE

This section provides a general summary of what this specificationdiscloses.

In a disclosed method, an article of a thermoplastic elastomer having atleast one thin dimension (e.g., a thickness or width of about 10 mm orless) is infused with a supercritical fluid in a pressurized container,then rapidly depressurized and heated either by immersion in a heatedfluid that can rapidly heat the article or with infrared or microwaveradiation to heat and foam the article.

The foamed article may then be used to make a molded product. In variousembodiments, the article that is foamed is a pellet, bead, particle, atape, a ribbon, a rope, a film, a strand, or a fiber.

In a further aspect, the foamed article may be annealed at a temperaturethat allows equilibration after foaming to allow increased modulus todevelop which helps maintain shape; the annealing is preferablyperformed immediately after it is foamed (i.e., before it is cooledsignificantly).

In a further aspect, the supercritical fluid comprises a polar liquid toadjust its Hildebrand solubility parameter to be nearer to that of thethermoplastic elastomer.

“A,” “an,” “the,” “at least one,” and “one or more” are usedinterchangeably to indicate that at least one of the item is present; aplurality of such items may be present unless the context clearlyindicates otherwise. All numerical values of parameters (e.g., ofquantities or conditions) in this specification, including the appendedclaims, are to be understood as being modified in all instances by theterm “about” whether or not “about” actually appears before thenumerical value. “About” indicates that the stated numerical valueallows some slight imprecision (with some approach to exactness in thevalue; approximately or reasonably close to the value; nearly). If theimprecision provided by “about” is not otherwise understood in the artwith this ordinary meaning, then “about” as used herein indicates atleast variations that may arise from ordinary methods of measuring andusing such parameters. In addition, disclosure of ranges are to beunderstood as specifically disclosing all values and further dividedranges within the range.

The terms “comprising,” “including,” and “having” are inclusive andtherefore specify the presence of stated features, steps, operations,elements, or components, but do not preclude the presence or addition ofone or more other features, steps, operations, elements, or components.Orders of steps, processes, and operations may be altered when possible,and additional or alternative steps may be employed. As used in thisspecification, the term “or” includes any one and all combinations ofthe associated listed items.

DETAILED DESCRIPTION

This section provides specific examples intended to illustrate theinvention that are not necessarily limiting as to materials andprocesses.

An article of a thermoplastic elastomer, preferably having at least onethin dimension (e.g., a thickness or width of about 10 mm or less,preferably 5 mm or less), is infused with a supercritical fluid in apressurized container, then rapidly depressurized and heated either byimmersion in a heated fluid or with infrared or microwave radiation tofoam the article.

The article that is foamed may have a regular or irregular shape and maybe, for example, a pellet, bead, particle, cylinder, prolate obloid,cube, sphere, pyramid, tape, ribbon, rope, film, strand, or fiber.Pellets, beads, or particles may be generally spherical, cylindricalellipsoidal, cubic, rectangular, and other generally polyhedral shapesas well as irregular or other shapes, including those having circular,elliptical, square, rectangular or other polygonal cross-sectional outerperimeter shapes or irregular cross-sectional shapes with or withoutuniform widths or diameters along an axis. “Generally” is used here toindicate an overall shape that may have imperfections andirregularities, such as bumps, dents, imperfectly aligned edges,corners, or sides, and so on.

The article is a thermoplastic elastomer. Nonlimiting examples ofsuitable thermoplastic elastomers include thermoplastic polyurethaneelastomers, thermoplastic polyurea elastomers, thermoplastic polyamideelastomers (PEBA or polyether block polyamides), thermoplastic polyesterelastomers, metallocene-catalyzed block copolymers of ethylene andα-olefins having 4 to about 8 carbon atoms, and styrene block copolymerelastomers such as poly(styrene-butadiene-styrene),poly(styrene-ethylene-co-butylene-styrene), andpoly(styrene-isoprene-styrene).

Thermoplastic polyurethane elastomers may be selected from thermoplasticpolyester-polyurethanes, polyether-polyurethanes, andpolycarbonate-polyurethanes, including, without limitation,polyurethanes polymerized using as polymeric diol reactants polyethersand polyesters including polycaprolactone polyesters. These polymericdiol-based polyurethanes are prepared by reaction of the polymeric diol(polyester diol, polyether diol, polycaprolactone diol,polytetrahydrofuran diol, or polycarbonate diol), one or morepolyisocyanates, and, optionally, one or more chain extension compounds.Chain extension compounds, as the term is being used, are compoundshaving two or more functional groups reactive with isocyanate groups,such as the diols, amino alcohols, and diamines. Preferably thepolymeric diol-based polyurethane is substantially linear (i.e.,substantially all of the reactants are difunctional).

Diisocyanates used in making the polyurethane elastomers may be aromaticor aliphatic. Useful diisocyanate compounds used to preparethermoplastic polyurethanes include, without limitation, isophoronediisocyanate (IPDI), methylene bis-4-cyclohexyl isocyanate (H₁₂MDI),cyclohexyl diisocyanate (CHDI), m-tetramethyl xylene diisocyanate(m-TMXDI), p-tetramethyl xylene diisocyanate (p-TMXDI), 4,4′-methylenediphenyl diisocyanate (MDI, also known as 4,4′-diphenylmethanediisocyanate), 2,4- or 2,6-toluene diisocyanate (TDI), ethylenediisocyanate, 1,2-diisocyanatopropane, 1,3-diisocyanatopropane,1,6-diisocyanatohexane (hexamethylene diisocyanate or HDI), 1,4-butylenediisocyanate, lysine diisocyanate, meta-xylylenediioscyanate andpara-xylylenediisocyanate, 4-chloro-1,3-phenylene diisocyanate,1,5-tetrahydro-naphthalene diisocyanate, 4,4′-dibenzyl diisocyanate, andxylylene diisocyanate (XDI), and combinations of these. Nonlimitingexamples of higher-functionality polyisocyanates that may be used inlimited amounts to produce branched thermoplastic polyurethanes(optionally along with monofunctional alcohols or monofunctionalisocyanates) include 1,2,4-benzene triisocyanate, 1,3,6-hexamethylenetriisocyanate, 1,6,11-undecane triisocyanate, bicycloheptanetriisocyanate, triphenylmethane-4,4′,4″-triisocyanate, isocyanurates ofdiisocyanates, biurets of diisocyanates, allophanates of diisocyanates,and the like.

Nonlimiting examples of suitable diols that may be used as extendersinclude ethylene glycol and lower oligomers of ethylene glycol includingdiethylene glycol, triethylene glycol and tetraethylene glycol;propylene glycol and lower oligomers of propylene glycol includingdipropylene glycol, tripropylene glycol and tetrapropylene glycol;cyclohexanedimethanol, 1,6-hexanediol, 2-ethyl-1,6-hexanediol,1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, 1,3-propanediol,butylene glycol, neopentyl glycol, dihydroxyalkylated aromatic compoundssuch as the bis(2-hydroxyethyl)ethers of hydroquinone and resorcinol;p-xylene-α,α′-diol; the bis(2-hydroxyethyl)ether of p-xylene-α,α′-diol;m-xylene-α,α′-diol and combinations of these. Thermoplasticpolyurethanes may be made using small amounts of triols or higherfunctionality polyols, such as trimethylolpropane or pentaerythritol,optionally along with monomeric alcohols such as C2-C8 monools ormonoisocyanates such as butyl isocyanate.

Useful active hydrogen-containing chain extension agents generallycontain at least two active hydrogen groups, for example, diols,dithiols, diamines, or compounds having a mixture of hydroxyl, thiol,and amine groups, such as alkanolamines, aminoalkyl mercaptans, andhydroxyalkyl mercaptans, among others. The molecular weight of the chainextenders preferably range from about 60 to about 400. Alcohols andamines are preferred. Examples of useful diols include those diolsalready mentioned. Suitable diamine extenders include, withoutlimitation, ethylene diamine, diethylene triamine, triethylenetetraamine, and combinations of these. Other typical chain extenders areamino alcohols such as ethanolamine, propanolamine, butanolamine, andcombinations of these. The dithiol and diamine reactants may also beincluded in preparing polyurethanes that are not elastomeric.

In addition to difunctional extenders, a small amount of a trifunctionalextender such as trimethylolpropane, 1,2,6-hexanetriol and glycerol, ormonofunctional active hydrogen compounds such as butanol or dimethylamine, may also be present. The amount of trifunctional extender ormonofunctional compound employed may be, for example, 5.0 equivalentpercent or less based on the total weight of the reaction product andactive hydrogen containing groups used.

The polyester diols used in forming a thermoplastic polyurethaneelastomer are in general prepared by the condensation polymerization ofone or more polyacid compounds and one or more polyol compounds.Preferably, the polyacid compounds and polyol compounds aredi-functional, i.e., diacid compounds and diols are used to preparesubstantially linear polyester diols, although minor amounts ofmonofunctional, tri-functional, and higher functionality materials(perhaps up to 5 mole percent) can be included to provide a slightlybranched, but uncrosslinked polyester polyol component. Suitabledicarboxylic acids include, without limitation, glutaric acid, succinicacid, malonic acid, oxalic acid, phthalic acid, hexahydrophthalic acid,adipic acid, maleic acid, suberic acid, azelaic acid, dodecanedioicacid, their anhydrides and polymerizable esters (e.g., methyl esters)and acid halides (e.g., acid chlorides), and mixtures of these. Suitablepolyols include those already mentioned, especially the diols. Inpreferred embodiments, the carboxylic acid component includes one ormore of adipic acid, suberic acid, azelaic acid, phthalic acid,dodecanedioic acid, or maleic acid (or the anhydrides or polymerizableesters of these) and the diol component includes one or more of includes1,4-butanediol, 1,6-hexanediol, 2,3-butanediol, or diethylene glycol.Typical catalysts for the esterification polymerization are protonicacids, Lewis acids, titanium alkoxides, and dialkyltin oxides.

A polymeric polyether or polycaprolactone diol reactant for preparingthermoplastic polyurethanes may be obtained by reacting a diolinitiator, e.g., 1,3-propanediol or ethylene or propylene glycol, with alactone or alkylene oxide chain-extension reagent. Lactones that can bering opened by an active hydrogen are well-known in the art. Examples ofsuitable lactones include, without limitation, ε-caprolactone,γ-caprolactone, β-butyrolactone, β-propriolactone, γ-butyrolactone,α-methyl-γ-butyrolactone, β-methyl-γ-butyrolactone, γ-valerolactone,δ-valerolactone, γ-decanolactone, δ-decanolactone, γ-nonanoic lactone,γ-octanoic lactone, and combinations of these. In one preferredembodiment, the lactone is ε-caprolactone. Useful catalysts includethose mentioned above for polyester synthesis. Alternatively, thereaction can be initiated by forming a sodium salt of the hydroxyl groupon the molecules that will react with the lactone ring.

In other embodiments, a diol initiator may be reacted with anoxirane-containing compound to produce a polyether diol to be used inthe polyurethane elastomer polymerization. Alkylene oxide polymersegments include, without limitation, the polymerization products ofethylene oxide, propylene oxide, 1,2-cyclohexene oxide, 1-butene oxide,2-butene oxide, 1-hexene oxide, tert-butylethylene oxide, phenylglycidyl ether, 1-decene oxide, isobutylene oxide, cyclopentene oxide,1-pentene oxide, and combinations of these. The oxirane-containingcompound is preferably selected from ethylene oxide, propylene oxide,butylene oxide, tetrahydrofuran, and combinations of these. The alkyleneoxide polymerization is typically base-catalyzed. The polymerization maybe carried out, for example, by charging the hydroxyl-functionalinitiator compound and a catalytic amount of caustic, such as potassiumhydroxide, sodium methoxide, or potassium tert-butoxide, and adding thealkylene oxide at a sufficient rate to keep the monomer available forreaction. Two or more different alkylene oxide monomers may be randomlycopolymerized by coincidental addition or polymerized in blocks bysequential addition. Homopolymers or copolymers of ethylene oxide orpropylene oxide are preferred. Tetrahydrofuran may be polymerized by acationic ring-opening reaction using such counterions as SbF₆ ⁻, AsF₆ ⁻,PF₆ ⁻, SbCl₆ ⁻, BF₄ ⁻, CF₃SO₃ ⁻, FSO₃ ⁻, and ClO₄ ⁻. Initiation is byformation of a tertiary oxonium ion. The polytetrahydrofuran segment canbe prepared as a “living polymer” and terminated by reaction with thehydroxyl group of a diol such as any of those mentioned above.Polytetrahydrofuran is also known as polytetramethylene ether glycol(PTMEG).

Aliphatic polycarbonate diols that may be used in making a thermoplasticpolyurethane elastomer are prepared by the reaction of diols withdialkyl carbonates (such as diethyl carbonate), diphenyl carbonate, ordioxolanones (such as cyclic carbonates having five- and six-memberrings) in the presence of catalysts like alkali metal, tin catalysts, ortitanium compounds. Useful diols include, without limitation, any ofthose already mentioned. Aromatic polycarbonates are usually preparedfrom reaction of bisphenols, e.g., bisphenol A, with phosgene ordiphenyl carbonate.

In various embodiments, the polymeric diol preferably has a weightaverage molecular weight of at least about 500, more preferably at leastabout 1000, and even more preferably at least about 1800 and a weightaverage molecular weight of up to about 10,000, but polymeric diolshaving weight average molecular weights of up to about 5000, especiallyup to about 4000, may also be preferred. The polymeric dioladvantageously has a weight average molecular weight in the range fromabout 500 to about 10,000, preferably from about 1000 to about 5000, andmore preferably from about 1500 to about 4000. The weight averagemolecular weights may be determined by ASTM D-4274.

The reaction of the polyisocyanate, polymeric diol, and diol or otherchain extension agent is typically carried out at an elevatedtemperature in the presence of a catalyst. Typical catalysts for thisreaction include organotin catalysts such as stannous octoate, dibutyltin dilaurate, dibutyl tin diacetate, dibutyl tin oxide, tertiaryamines, zinc salts, and manganese salts. Generally, for elastomericpolyurethanes, the ratio of polymeric diol, such as polyester diol, toextender can be varied within a relatively wide range depending largelyon the desired hardness of the final polyurethane elastomer. Forexample, the equivalent proportion of polyester diol to extender may bewithin the range of 1:0 to 1:12 and, more preferably, from 1:1 to 1:8.Preferably, the diisocyanate(s) employed are proportioned such that theoverall ratio of equivalents of isocyanate to equivalents of activehydrogen containing materials is within the range of 1:1 to 1:1.05, andmore preferably, 1:1 to 1:1.02. The polymeric diol segments typicallyare from about 35% to about 65% by weight of the polyurethane polymer,and preferably from about 35% to about 50% by weight of the polyurethanepolymer.

The selection of diisocyanate, extenders, polymeric diols, and theweight percent of the polymeric diols used takes into account thedesired density and stability of the finished foam. In general, agreater content of a polymeric polyol that has a Hildenbrand solubilityparameter closer to that of the supercritical fluid will permit higherabsorption of the supercritical fluid that results in a lower densityfoam. Also in general, shorter polymeric diols provide foams that shrinkless after they are first foamed. Use of higher number average molecularweight polymeric diols allow a higher degree of swelling, but amolecular weight that is too high may yield a less stable foam.

Suitable thermoplastic polyurea elastomers may be prepared by reactionof one or more polymeric diamines or polyols with one or more of thepolyisocyanates already mentioned and one or more diamine extenders.Nonlimiting examples of suitable diamine extenders include ethylenediamine, 1,3-propylene diamine, 2-methyl-pentamethylene diamine,hexamethylene diamine, 2,2,4- and 2,4,4-trimethyl-1,6-hexane diamine,imino-bis(propylamine), imido-bis(propylamine),N-(3-aminopropyl)-N-methyl-1,3-propanediamine),1,4-bis(3-aminopropoxy)butane, diethyleneglycol-di(aminopropyl)ether),1-methyl-2,6-diamino-cyclohexane, 1,4-diamino-cyclohexane, 1,3- or1,4-bis(methylamino)-cyclohexane, isophorone diamine, 1,2- or1,4-bis(sec-butylamino)-cyclohexane, N,N′-diisopropyl-isophoronediamine, 4,4′-diamino-dicyclohexylmethane,3,3′-dimethyl-4,4′-diamino-dicyclohexylmethane,N,N′-dialkylamino-dicyclohexylmethane, and3,3′-diethyl-5,5′-dimethyl-4,4′-diamino-dicyclohexylmethane. Polymericdiamines include polyoxyethylene diamines, polyoxypropylene diamines,poly(oxyethylene-oxypropylene)diamines, and poly(tetramethyleneether)diamines. The amine- and hydroxyl-functional extenders alreadymentioned may be used as well. Generally, as before, trifunctionalreactants are limited and may be used in conjunction with monofunctionalreactants to prevent crosslinking.

Suitable thermoplastic polyamide elastomers may be obtained by: (1)polycondensation of (a) a dicarboxylic acid, such as oxalic acid, adipicacid, sebacic acid, terephthalic acid, isophthalic acid,1,4-cyclohexanedicarboxylic acid, or any of the other dicarboxylic acidsalready mentioned with (b) a diamine, such as ethylenediamine,tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, ordecamethylenediamine, 1,4-cyclohexanediamine, m-xylylenediamine, or anyof the other diamines already mentioned; (2) a ring-openingpolymerization of a cyclic lactam, such as ε-caprolactam orω-laurolactam; (3) polycondensation of an aminocarboxylic acid, such as6-aminocaproic acid, 9-aminononanoic acid, 11-aminoundecanoic acid, or12-aminododecanoic acid; or (4) copolymerization of a cyclic lactam witha dicarboxylic acid and a diamine to prepare a carboxylicacid-functional polyamide block, followed by reaction with a polymericether diol (polyoxyalkylene glycol) such as any of those alreadymentioned. Polymerization may be carried out, for example, attemperatures of from about 180° C. to about 300° C. Specific examples ofsuitable polyamide blocks include NYLON 6, NYLON 66, NYLON 610, NYLON11, NYLON 12, copolymerized NYLON, NYLON MXD6, and NYLON 46.

The effects of the type and molecular weights of the soft segmentpolymeric polyols used in making thermoplastic polyurea elastomers andpolyamide elastomers are analogous to the same effects in makingthermoplastic polyurethane elastomers.

Thermoplastic polyester elastomers have blocks of monomer units with lowchain length that form the crystalline regions and blocks of softeningsegments with monomer units having relatively higher chain lengths.Thermoplastic polyester elastomers are commercially available under thetradename HYTREL from DuPont.

Metallocene-catalyzed block copolymers of ethylene and α-olefins having4 to about 8 carbon atoms are prepared by single-site metallocenecatalysis of ethylene with a softening comonomer such as hexane-1 oroctene-1, for example in a high pressure process in the presence of acatalyst system comprising a cyclopentadienyl-transition metal compoundand an alumoxane. Octene-1 is a preferred comonomer to use. Thesematerials are commercially available from ExxonMobil under the tradenameExact™ and from the Dow Chemical Company under the tradename Engage™.

Styrene block copolymer elastomers such aspoly(styrene-butadiene-styrene),poly(styrene-ethylene-co-butylene-styrene), andpoly(styrene-isoprene-styrene) may be prepared may anionicpolymerization in which the polymer segments are produced sequentially,first by reaction of an alkyl-lithium initiator with styrene, thencontinuing polymerization by adding the alkene monomer, then completingpolymerization by again adding styrene. S-EB-S and S-EP-S blockcopolymers are produced by hydrogenation of S-B-S and S-I-S blockcopolymers, respectively.

The thermoplastic elastomers may be formed into a pellet, bead,particle, cylinder, prolate obloid, cube, spheres, pyramids, tape,ribbon, rope, film, strand, or fiber by known methods, such as extrusionthrough an appropriately shaped die, cooling, and cutting, or othercommon forming methods such as compression molding, casting,thermoforming, injection molding, blow molding, or transfer compressionmolding. Any article made by any of these processes, or optionallypost-processed or subjected to ultrasonic RF welding or thermal weldingbefore being foamed in the disclosed foaming process. In variousembodiments, the article is a preform. The article may also be a tape,strand, fiber, or other article that is relatively long compared to itsother dimensions, which may be laid up before being infused with thesupercritical fluid, then foamed.

Thermoplastic elastomer articles of a desired shape are infused,preferably to saturation, with a supercritical fluid, which in manyembodiments is preferably supercritical carbon dioxide.

Nonlimiting examples of suitable compounds that can be used as thesupercritical fluid include carbon dioxide (critical temperature 31.1°C., critical pressure 7.38 MPa), nitrous oxide (critical temperature36.5° C., critical pressure 7.24 MPa), ethane (critical temperature32.3° C., critical pressure 4.88 MPa), ethylene (critical temperature9.3° C., critical pressure 5.12 MPa), nitrogen (critical temperature−147° C., critical pressure 3.39 MPa), and oxygen (critical temperature−118.6° C., critical pressure 5.08 MPa).

Carbon dioxide is often used as a supercritical fluid in differentprocesses. The supercritical carbon dioxide fluid can be made morecompatible with the polar thermoplastic elastomers (particularlythermoplastic polyurethane, polyurea, and polyamide elastomers) bymixing it with a polar fluid such as methanol, ethanol, propanol, orisopropanol. The polar fluid that is used should have a Hildebrandsolubility parameter equal to or greater than 9 MPa^(−1/2). Increasingthe weight fraction of the polar fluid increases the amount of carbondioxide uptake, but the polar fluid is also taken up, and at some pointthere is a shift from a maximum amount of uptake of the supercriticalcarbon dioxide to an increasing amount of the non-foaming agent polarfluid being taken up by the thermoplastic elastomer article. In certainembodiments, from about 0.1 mole % to about 7 mole % of the polar fluidis included in the supercritical fluid, based on total fluid, especiallywhen used to infuse a polyurethane elastomer, polyurea elastomer, or apolyamide elastomer.

Supercritical fluids may be used in combination. In some cases,supercritical nitrogen may be used as a nucleating agent in a smallweight percentage along with supercritical carbon dioxide or anothersupercritical fluid that acts as the blowing agent. Nano-sized particlessuch as nano clays, carbon black, crystalline, immiscible polymers, andinorganic crystals such as salts can be included as nucleating agents.

The articles are placed in a vessel that can withstand high pressure.The vessel is closed and CO₂ or other type of foaming agent isintroduced. The vessel temperature and pressure are maintained above thecritical temperature and pressure of the foaming agent. Once the articleis saturated with the foaming agent, the vessel is rapidly depressurized(the depressurizing process can last up to a minute or so). The articleis then removed from the vessel and heated to produce the foamed part.When a co-solvent is used, it can be introduced along with the CO₂ oradded to the vessel with the article before the vessel is closed.

The thermoplastic article is soaked in the supercritical fluid underconditions—temperature and pressure—and for a time to allow it to takeup a desired amount of the supercritical fluid.

In various embodiments, the thermoplastic article is soaked underconditions that result in it becoming saturated with the supercriticalfluid. The article is then removed from the chamber and immediatelyeither heated to a temperature in a medium with suitable thermalcharacteristics for foaming to occur or is exposed to microwaves orinfrared radiation in a tunnel or oven to cause the foaming to occur. Inmicrowave heating, the material is exposed to an electromagnetic wavethat causes the molecules in the material to oscillate, therebygenerating heat. The system can be designed to work in batch orcontinuous process. In a batch process, the articles saturated with thesupercritical fluid are placed in a microwave oven or a device equippedwith an IR lamp or IR lamps. Preferably the articles are rotated oragitated, when their size is small enough, to ensure fast and uniformheating. When foaming is completed, the articles are removed from thesystem. The heating can also be done in the continuous process. Thearticles are placed on a planar surface such as a belt that moves themthrough a tunnel or through a pipe. The system is designed so that theheating elements (IR lamp or microwave generator) can apply power toachieve rapid uniform heating. The time of heating is controlled by thespeed by which the articles move through the tunnel or pipe.

Water is one suitable medium in which foaming readily occurs at anappropriate temperature because water has a high heat capacity and heattransfer rate. In certain preferred embodiments, the thermoplasticelastomer article infused or saturated with supercritical fluid issubmerged in water that is at a temperature at least about 80° higherand, preferably, at least about 100° higher than the elastomer's (softsegment) T_(g) but less than the elastomer's (hard segment) T_(m).

Other suitable mediums are steam or pressurized hot air.

Time, temperature, and pressure in the step of solvating thethermoplastic elastomer article with the supercritical fluid and thedepressurization rate, temperature, and medium in the foaming step allaffect the degree of foaming achieved. In general, a thicker articlemust be kept in the supercritical fluid for a longer time to becomesaturated with the supercritical fluid. A sheet of thermoplasticpolyurethane elastomer (made from SP9213, obtained from BASFCorporation) having a thickness of 25 mm was cut to 25 mm×64 mm samplepieces. The samples were soaked in supercritical carbon dioxide underconditions as noted in Table 1. The vessel was then depressurized andthe samples were rapidly transferred to a water bath at the temperatureindicated in Table 1 for foaming. The percentage growth was calculatedas:(area of the foam sheet−area f the original sample)/area of the originalsample

The foam sheets were measured three times: fresh out of the water bath,one day later, and one week later. All foams showed some degree ofshrinkage in the first 24 hours and very little shrinkage afterwards.

Supercritical Supercritical 1 Week 1 Week Fluid Fluid Water BeforeBefore after after Soak time, Pressure, Temperature, Temperature,thickness, length, width, length, Example minute MPa ° C. ° C. mm mm mmmm Expansion, % 1 10 10 25 50 25 65 32 77 51 2 10 10 50 90 25 65 33 8574 3 1440 10 50 50 25 65 26 64 2 4 1440 10 25 90 25 65 46 114 222 5 1050 50 50 25 65 25 65 1 6 10 50 25 90 25 65 49 121 266 7 1440 50 25 50 2565 28 71 23 8 1440 50 50 90 25 65 37 88 102

The foamed article may be annealed at an elevated temperature after thefoaming process. While not wishing to be bound by theory, it is believedthat annealing the article may allow phase segregation of the elastomersthat are placed under strain, e.g. the mold, and stress, a partialpressure external to moderate internal pressure equilibration just afterrapid foaming. Cooling under balanced forces allow the increased modulusto maintain shape once at room temperature and atmospheric pressure.

The article may be annealed at a temperature from above ambient to justbelow the T_(m) of the thermoplastic elastomer (which may be determineby the usual appropriate thermal methods, of which differential scanningcalorimetry (DSC) may be mentioned) for a time sufficient to stabilizethe foam.

In various embodiments, the foamed articles prepared by the disclosedmethod are further molded or shaped. In one method, the foamed articlesare beads, pellets, particles, or similar relatively small sizes, whichwill be generally referred to in the following discussion as ‘pellets.’A mold is filled with the foamed pellets and the pellets are molded atan appropriate temperature into a shaped article. The shaped article maybe of any dimensions. For example, the molded article may be sized as acushion or cushioning element that can be included in an article offootwear, for example part of a footwear upper, such as a foam elementin a collar or tongue, as an insole, as a midsole or a part of amidsole, or an outsole or a part of an outsole; foam padding in shinguards, shoulder pads, chest protectors, masks, helmets or otherheadgear, knee protectors, and other protective equipment; an elementplaced in an article of clothing between textile layers; in clothing, inprotective gear such as helmets, chest protectors, and shoulder pads, ormay be used for other known padding applications for protection orcomfort, especially those for which weight of the padding is a concern;or in furniture or in seats, for example bicycle seats.

In one embodiment, a foamed article, such as a midsole for footwear, isformed by placing desired amount of thermoplastic polyurethane foamedpellets in a compression mold in the shape of the article and the moldis brought to a peak temperature of from about 100° C. to about 180° C.over a period of from about 300 to about 1500 seconds, then cooled tofrom about 5° C. to about 80° C. over a period of from about 300 toabout 1500 seconds within about 30 seconds after the peak temperature isreached. In various embodiments, the thermoplastic polyurethane foampellets may preferably be generally spherical or ellipsoidal. In thecase of non-spherical pellets, for example ellipsoidal beads, thelargest major diameter of a cross-section taken perpendicular to themajor (longest) axis of the ellipsoid. The foam pellets may preferablyhave a diameter of from about 0.5 mm to about 1.5 cm. Ellipsoidalpellets may be from about 2 mm to about 20 mm in length and from about 1to about 20 mm in diameter. Each individual pellet may be, for example,from about 20 to about 45 mg in weight. The foam pellets may have adensity of from about 0.01 to about 0.3 g/cm³ and the molded article mayhave a density from about 0.1 to about 0.45 g/cm³.

A desired amount of the thermoplastic polyurethane foam pellets areplaced in the compression mold. The foamed pellets may be placed in themold when both the mold and the foamed pellets are at a temperaturebelow about 80° C. Preferably, the temperatures of the mold and of thefoamed beads are both ambient temperature (about 5-27° C.), although asmentioned the temperatures of each may be higher, up to perhaps 80° C.

The foam pellets may be coated with an adhesive before being placed inthe mold. Suitable adhesives include W-104, W-105, W-01, W-01S and SW07from Henkel. Other adhesives such as WA-1C and WP 1-116K from Han YoungIndustry Company can also be used. In general, these adhesives may besprayed onto the foamed pellets or otherwise coated onto the foamedpellets.

The mold is brought to a peak temperature that is in the range of upabout 110° C. over a period of from about 300 to about 1500 seconds. Ingeneral, a longer time may be used for heating a thicker part to moldthe part. Thus, a thicker part may be brought to the peak moldingtemperature over a longer period of time compared to the time in which athinner part is brought to the peak molding temperature. In variousembodiments, the mold is brought to the peak temperature over a periodof from about 300 to about 1200 seconds or from about 300 to about 900seconds. A desired skin thickness may be achieved by selection of themaximum heating temperature within the temperature range. Skin thicknessmay be selected to alter cushioning and feel of a molded midsole as usedin an article of footwear. The skin thickness on a bead may be about 10micrometers. The skin thickness on a molded part may be at least about20 micrometers. In various embodiments, the peak temperature is selectedto produce a skin thickness of from about 10 to about 200 micrometers.

The mold is then cooled to a temperature of from about 5° C. to about80° C. over a period of from about 300 to about 1500 seconds. Cooling istypically carried out by moving the mold to the cold side of thecompression molding press between two cold plates. In general, a longertime may be used for cooling a thicker part.

In other embodiments, the foamed pellets are molded with a matrixmaterial of an unfoamed thermoplastic elastomer, which may include ablowing agent so that it is foamed during the molding process.

The molded article may be used as an insert in a further moldingprocess, such as in a thermoforming process.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention. Individual elements or features ofa particular embodiment are generally not limited to that particularembodiment, but can be used in other embodiments and can be combined inother ways, even if not specifically shown or described. Such variationsare included in the invention.

What is claimed is:
 1. A method of foaming an article comprising athermoplastic elastomer, comprising: infusing the article with asupercritical fluid; removing the article from the supercritical fluidand either (i) immersing the article in a heated fluid or (ii)irradiating the article with infrared or microwave radiation to make afoamed article, wherein the supercritical fluid comprises a polar fluidhaving a Hildebrand solubility parameter equal to or greater than 9MPa^(−1/2), then annealing the foamed article.
 2. The method accordingto claim 1, wherein the thermoplastic elastomer is selected from thegroup consisting of thermoplastic polyurethane elastomers, thermoplasticpolyurea elastomers, thermoplastic polyamide elastomers,metallocene-catalyzed block copolymer elastomers of ethylene andα-olefins having 4 to about 8 carbon atoms, styrene block copolymerelastomers, and combinations thereof.
 3. The method according to claim2, wherein the thermoplastic elastomer comprises a thermoplasticpolyurethane elastomer.
 4. The method according to claim 1, wherein thearticle is in a shape of a pellet, bead, particle, tape, ribbon, rope,film, strand, or fiber.
 5. The method according to claim 4, wherein thearticle has at least one dimension that is 10 mm or less.
 6. The methodaccording to claim 1, wherein the article is saturated with thesupercritical fluid.
 7. The method according to claim 1, wherein thesupercritical fluid comprises carbon dioxide.
 8. The method according toclaim 1, wherein the supercritical fluid comprises nitrogen.
 9. Themethod according to claim 1, wherein the article is immersed in a heatedfluid after being removed from the supercritical fluid to cause thearticle to foam, the fluid being at a temperature that is at least about80° higher than the elastomer's T_(g) but less than the elastomer'sT_(m).
 10. The method according to claim 9, wherein the heated fluid iswater.
 11. The method according to claim 1, wherein the article isheated with microwaves or infrared radiation after being removed fromthe supercritical fluid to cause the article to foam.
 12. The methodaccording to claim 1, wherein the foamed article is annealed at atemperature below the elastomers T_(m).
 13. The method according toclaim 1, further comprising molding the foamed article.
 14. The methodaccording to claim 13, wherein the foamed article is a pellet, bead, orparticle and wherein a plurality of the articles are molded together.15. The method according to claim 14, wherein the plurality of thearticles are coated with an adhesive before being molded together. 16.The method according to claim 14, wherein the plurality of the articlesare molded together in a matrix of unfoamed thermoplastic elastomer. 17.The method according to claim 14, wherein the plurality of the articlesare molded together in a matrix of foamed thermoplastic elastomer.